If you go to a TSC or Rural Kind or Orshelean's (sp?) store...any store that sells horse take you can make you're own very easily. Look for a cinch strap made of the thickest smoothest leather that's there. No it's not fancy leather but by the time the TYPE of leather starts to matter you won't be needing to ask questions.
I can make three long hanging strops for $18. If fact, I have four hanging from the towel peg on my bathroom door right now. 3 with compounds and one "bare". (I use these things for my straight razors...so I'm serious about not needing to get picky about the TYPE of leather)
The cinch straps will have holes already punched in them...that's ok. I actually use those holes to tie a piece of rope through to make them hanging. Of course you can just cut those sections out if you want to glue it to a board.
Don't let the leather dry out. I just rub my thumb across my nose and then spread it out across the leather. Gross, maybe...but the price is right, it's quick, and it's always handy.
The cardboard works great, the paper works great (nobody's mentioned newspaper yet...it works very well), the window edge works, and the bottom of a can of copenhagen can even do well in a pinch. {I crack myself up, forgive me.}
If you want to get into charging compounds PM me. You can get some that are very very very fine...there aren't any stones I know of finer than 30,000 grit. They cost nearly 3 days wage for me...or you can get some chromium oxide (0.5 microns, roughly the same as 30K) for less than an hours wage.